Scientists still say low oxygen levels caused the massive fish die-off but suggest domoic acid could have caused them to swim into King Harbor.

Sardines that suffocated and died en masse this week in King Harbor have tested positive for a powerful neurotoxin that scientists believe may have distressed 1 million or more fish off the Los Angeles coastline and caused them to swim chaotically into the Redondo Beach marina.

Researchers still believe critically low oxygen levels, not the toxin or an algae bloom, caused the fish to suddenly die Monday night in the Redondo Beach marina.

But the discovery of domoic acid in dead fish — reported Friday by USC biologists — could help explain why millions of sardines swam into the harbor in the first place.

"It is possible that high levels of domoic acid in the sardines in King Harbor may have exacerbated physiological stress of the fish brought on by oxygen depletion of the water, or may have been a contributing explanation for them congregating in the harbor at very high abundances, but this has not been confirmed," USC professor David Caron wrote Friday in a summary of his laboratory's findings.

Domoic acid is often found in the stomachs of fish that have been feeding on plankton on the ocean's surface during toxic algae blooms. The toxin has been linked to neurological disorders, illnesses and deaths of seabirds, sea lions, sea otters and whales. When it accumulates in edible fish and shellfish, it can sicken humans.

Caron's lab, which began studying King Harbor after a fish die-off at the marina in 2005, is working to determine if the poisoning was caused by a toxic algae bloom spotted Wednesday about 12 miles southwest of Redondo Beach.

The presence of the toxin in the fish could lead to health complications for seabirds that were seen eating the dead fish that carpeted the harbor surface Tuesday morning.

"There were tons of birds feeding on these fish and it's conceivable that we'll see some bird mortality as a result," Caron said.

More than 35 tons of dead sardines have been scooped, pulled or vacuumed from the marina, but officials believe an equal number may have settled on the ocean floor. Workers are racing to cleanse the harbor, fearing that if the dead fish are left to decompose, they could trigger an algae bloom.

Scientists are also looking into another theory: that a pulse of oxygen-starved ocean water swept into the harbor just before the die-off and worsened conditions for the fish.

The California Department of Fish and Game has blamed the massive fish die-off on oxygen deprivation and is also testing fish for toxins at its animal forensics laboratory near Sacramento, where results are not expected until next week.

"It's going to be a full-on CSI thing with these fish," spokesman Andrew Hughan said. "We're trying to find out what drove them and why they swam in there. There's the predator theory, there's the rough-water theory; it could be anything, and there may never be a definitive answer."